1. Field of the Invention
The present invention relates to a method and apparatus for performing optical serial-to-parallel data conversion. Amongst other things, the present invention utilizes second-harmonic generation to perform high-speed optical time-domain demultiplexing, optical code recognition and serial-to-parallel data conversion.
2. Background
Fiber-optic communication technology is being developed and commercialized at virtually unprecedented rates. It can be shown that, for transmission links on the order of kilometers, commercially available fibers can support data rates exceeding 100 Gb/s on a single optical carrier in the 1.3-1.6 micron wavelength range without resorting to operation at exactly the zero-dispersion wavelength. However, at these data rates, electronic components have difficulty generating, interpreting and/or switching the data. That is, the data rate conversion and manipulation has yet to catch up to the data transmission capabilities.
In view of the limitations of electronics, it is evident that to implement high-speed communication systems it is necessary to have a high-speed optical signal processing technology to compliment the high-speed optical-transmission technology. In addition, new system concepts which are compatible with the higher data rates and with the optical hardware should be developed.
One such concept being used to develop hardware to mate electrical and optical signal processing is the utilization of second-harmonic generation (SHG) and waveguide SHG in particular. Investigators Normandin and Stegeman have authored a number of papers detailing the occurrence of waveguide SHG. See for example: R. Normandin and G. I. Stegeman, xe2x80x9cNon-Degenerate Four-Wave Mixing in Integrated Optics,xe2x80x9d Optics Letters, Vol. 4, No. 2, February 1979; xe2x80x9cPicosecond Signal Processing with Planar, Nonlinear Integrated Optics,xe2x80x9d Applied Physics Letters, Vol. 36, No. 4, Feb. 15, 1980; P. J. Vella, R. Normandin, and G. I. Stegeman, xe2x80x9cEnhanced Second-Harmonic Generation by Counter-Propagating Guided Optical Waves,xe2x80x9d Applied Physics Letters, Vol. 38, No. 10, May 15, 1981; and, R. Normandin, S. Lxc3xa9toumeau, F. Chatenoud, and R. L. Williams, xe2x80x9cMonolithic, Surface-Emitting, Semiconductor Visible Lasers and Spectrometers for WDM Fiber Communication Systems,xe2x80x9d IEEE Journal of Quantum Electronics, Vol. 27, No. 6, June 1991. Normandin and Stegeman demonstrated waveguide SHG in strongly nonlinear optical materials by inserting an optical pulse at one end of the waveguide and another optical pulse at the other end of the waveguide. When the two injected fundamental signals collided, they produced a second-harmonic wave that propagated perpendicular to the waveguide surface. A serial-to-parallel converter device that utilizes SHG for the purposes of data communication is disclosed in U.S. Pat. No. 5,172,258 (the ""258 patent).
However, it is desirable to increase the efficiency of the second-harmonic conversion inside the waveguide of the serial-to-parallel converter of the ""258 patent in order to make the converter compatible with the small pulse energies required for greater than 100 Gb/s fiber-optic transmission. Additionally, a problem associated with nonlinear optical waveguides, such as those used for SHG, is the existence of two-photon absorption, which reduces the conversion efficiency by absorbing photons at the fundamental wavelength. The effects of two-photon absorption increase as the photon energy of light provided to the input of the waveguide approaches and passes through the center of the band gap of the material utilized for the waveguide. Finally, input power is often lost due to less than optimum coupling between a fiber optic element communicating an input optical serial data pulse stream to the waveguide; see, for example, V. Vusirikala, S. S. Saini, R. E. Bartolo, M. Dagenais, and D. R. Stone, xe2x80x9cCompact Mode Expanders Using Resonant Coupling Between a Tapered Active Region and an Underlying Coupling Waveguide,xe2x80x9d IEEE Photonics Technology Letters, Vol. 10, No. 2, February 1998.
Therefore, there is a need in the industry for a serial-to-parallel converter capable of optically converting a serial optical digital input signal into a set of parallel optical digital signals which addresses these and other related, and unrelated, problems.
The present invention allows the use of high-speed optical communication lines by efficiently converting a serial optical digital signal into a set of parallel optical digital signals. In one aspect, the present invention uses waveguide SHG to convert high-data-rate optical signals to lower data rates that are compatible with conventional high-speed electronic signal processing. The use of waveguide SHG also allows asynchronous operation, thereby greatly reducing circuit complexity as compared to conventional electronic methods of time demultiplexing or code recognition because clock recovery and synchronization can be done at the lower, demultiplexed data rate rather than at the greater than 100 Gb/s multiplexed rate.
According to a preferred embodiment of the present invention, second-harmonic photons are generated in the channel of a waveguide of an optical serial-to-parallel converter when each data pulse in an input serial optical data pulse stream collides with a single counter-propagating timing pulse. As currently conceived, half of the energy of the input optical stream is in the timing pulse, with the remainder divided equally among the data pulses. When the timing pulse reflects at the mirrored end of the waveguide, the timing pulse effectively counter-propagates through itself. This produces a comparatively large second-harmonic signal that is distinguishable from the reflections of the data pulses, and thus may be used as a trigger for other operations such as clock recovery and synchronization.
The collision between the timing pulse and each individual data pulse in the serial optical data pulse stream occurs at a predetermined, unique location in the waveguide. The SHG radiation generated by each collision travels in a direction which is perpendicular to the waveguide""s longitudinal axis and parallel to the SHG radiation produced by other collisions. By placing a fiber optic element of a fiber optic array or a photodetector of a photodetector array above the location of each collision (i.e., at the waveguide""s output), the serial-to-parallel converter derives a plurality of parallel output channels. In the case of fiber optic elements, the parallel output channels include a plurality of parallel optical data pulse streams. In the case of photodetectors, the parallel output channels include a plurality of parallel electrical signals.
In the preferred embodiment, a first reflector is positioned at a location substantially opposed to the waveguide output at the bottom of the waveguide""s channel. The first reflector is, preferably, aligned to reflect a portion of the second-harmonic radiation propagating away from the waveguide output. A second reflector is interposed at a location between the first reflector and the waveguide output. Preferably, the second reflector is located on the exit surface of the waveguide at the waveguide""s output. The second reflector selectively reflects a portion of the second-harmonic radiation propagating toward the waveguide output. Cooperating together to define a vertical resonant cavity in the waveguide that is resonant at the wavelength of the second-harmonic radiation, the first and second reflectors direct at least a portion of the plurality of photons of the second-harmonic radiation from each timing pulse-data pulse collision through the resonant cavity more than once during the time at which the collision occurs, thereby causing the generation of additional second-harmonic light. By generating additional second-harmonic light from each timing pulse-data pulse collision, the efficiency of the optical serial-to-parallel converter is increased. For the approximately 1 ps pulses required for greater than 100 Gb/s operation, the efficiency of the optical serial-to-parallel converter with the vertical resonant cavity is enhanced by one to two orders of magnitude over that of an optical serial-to-parallel converter without a resonant cavity. The increase in efficiency enables the device to be used in conjunction with an optical pre-amplifier to demultiplex data at power levels found in typical fiber optic communication systems.
In another embodiment of the present invention, an optical serial-to-parallel converter comprises a waveguide having a core interposed between first and second cladding. The core includes a first plurality of material layers having a high nonlinearity and a second plurality of material layers having a nonlinearity that is substantially less than the nonlinearity of the first plurality of material layers. The material layers of the first plurality of material layers and the second plurality of material layers are arranged in an alternating manner so that no material layer of the first plurality of material layers is adjacent to another material layer of the first plurality of material layers. Similarly, no material layer of the second plurality of material layers is adjacent to another material layer of the second plurality of material layers. Preferably, the material layers of the first and second pluralities of material layers include an aluminum gallium arsenide alloy. Preferably, the content, or concentration, of aluminum in the aluminum gallium arsenide of the material layers of the first and second pluralities of material layers is, respectively, in the ranges of twenty to thirty-five percent and eighty to ninety percent. By using an aluminum gallium arsenide alloy with a twenty to thirty-five percent aluminum content instead of gallium arsenide, the two-photon absorption coefficient in the waveguide is reduced from approximately 8 cm/GW to less than 0.5 cm/GW at a fundamental, or input, light wavelength of 1.5 microns (i.e., which is used for fiber optic communications), thereby increasing the efficiency of the converter. In addition, use of an aluminum gallium arsenide alloy with a twenty to thirty-five percent aluminum content decreases the waveguide""s linear absorption of the second-harmonic light generated by the timing pulse-data pulse collisions.
According to still another embodiment of the present invention, an optical serial-to-parallel converter comprises a spot size converter which is optically interposed between an fiber optic element communicating an input serial optical data pulse stream to the converter and the input of the converter""s waveguide. Use of a spot size converter enables the substantial matching of the mode-diameter of the waveguide""s input and the mode-diameter of the fiber optic element, thereby reducing the loss of power input to the waveguide and, hence, improving the overall efficiency of the converter. Substantial matching of the mode-diameters of the waveguide""s input and the fiber optic element also relaxes the alignment tolerances for the waveguide and fiber optic. Spot size converters are also employable at the waveguide""s output to substantially match the mode diameter at the waveguide""s output to the fiber optic elements of a fiber array with materially the same benefits.
One application of a serial-to-parallel converter of the present invention is time-domain demultiplexing. Where the input to the converter is a time-domain multiplexed (TDM) data stream comprised of a series of data frames, each data frame constituting one epoch of a TDM bit sequence, then each of the parallel output channels will carry demultiplexed data from the single input serial data stream.
Another application of a serial-to-parallel converter of the present invention is optical code recognition. In this case, the input to the converter is a binary code word represented by a series of optical pulses. The parallel output channels are switched on or off to represent a particular code word, and the resulting output is integrated (summed) and passed through an electronic threshold detector to determine if a given input signal matches the selected code word.
Another application of a serial-to-parallel converter of the present invention is serial-to-parallel data conversion. In this case, the serial-to-parallel converter functions exactly as for TDM demultiplexing. The primary difference is that the input serial data stream is created by multiplexing parallel channels from a single source (e.g. a high-speed computer bus) instead of multiple channels from different sources.
It is therefore an object of the present invention to convert an input serial optical data pulse stream into a plurality of output parallel optical data streams.
Another object of the present invention is to increase the efficiency of a serial-to-parallel converter for optical data by causing the generation of additional second-harmonic light.
Still another object of the present invention is to enable the use of a serial-to-parallel converter for optical data with an optical pre-amplifier to demultiplex data at power levels found in typical fiber optic communication systems.
Still another object of the present invention is to reduce the two-photon absorption in the waveguide of a serial-to-parallel converter for optical data.
Still another object of the present invention is to decrease the linear absorption of second-harmonic light generated by timing pulse-data pulse collisions in the waveguide of a serial-to-parallel converter for optical data.
These and other objects, features, advantages, and applications of the present invention will become apparent upon reading and understanding this specification, taken in conjunction with the accompanying drawings.